1. Field of the Invention
The present invention relates to a double hetero type epitaxial wafer, and more particularly, to a double hetero type epitaxial wafer suitable for producing a light emitting diode (LED) having a large light emission power output.
2. Description of the Related Art
In this specification, a compound consisting of a IIIb group element and a V.sub.b group element in the periodic table is referred to as a III-V group compound. A III-V group compound mixed crystal epitaxial wafer having a double hetero structure is suitable for producing an LED having a large power output, since it can confine carriers in an active layer and it has a low carrier loss due to a non-radiative recombination process.
Particularly, since by using an epitaxial wafer of a gallium aluminum arsenide (GaAlAs,) mixed crystal a red LED having a large power output can be produced, the epitaxial wafer is widely used for a display device or an automatic focus measuring device of a camera, etc.
The epitaxial wafer can be produced by an epitaxial growth of a required III-V group compound mixed crystal, which growth is carried out by a liquid phase epitaxial growth process, a chemical vapour epitaxial growth process, etc. on a single crystal substrate (hereinafter referred to as a substrate of gallium arsenide (GaAs), etc.) Nevertheless, since the GaAs, etc., used as a substrate has a low light penetration, light generated at a pn junction is absorbed by the substrate before being emitted to the outside. Conventionally, to prevent this light absorption by the substrate the substrate was cleaned by polishing or etching after the epitaxial growth.
Preferably, an epitaxial wafer has a thickness of at least 0.1 mm or more, to prevent a breakage of the wafer during the production of an LED. Therefore, the epitaxial layer is grown to a thickness of 0.1 mm or more.
When the thickness of the epitaxial layer is made thicker, however, a nonuniform mixed crystal composition is generated in an epitaxial layer particularly the clad layers, due to a difference of the segregation coefficient thereof.